1. Field of the Invention
The present invention relates to printers and plotters. More specifically, the present invention relates to inkjet printers and plotters having multiple pens for multi-color operation.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Inkjet printer/plotters, such as those sold by Hewlett Packard Company, offer substantial improvements in speed over the conventional X-Y plotter. Inkjet printer/plotters typically include a pen having an array of nozzles. The pens are mounted on a carriage which is moved across the page in successive swaths. Each inkjet pen has heater circuits which, when activated, cause ink to be ejected from associated nozzles. As the pen is positioned over a given location, a jet of ink is ejected from the nozzle to provide a pixel of ink at a desired location. The mosaic of pixels thus created provides a desired composite image.
Inkjet technology is now well known in the art. See, for example, U.S. Pat. Nos. 4,872,027, entitled PRINTER HAVING IDENTIFIABLE INTERCHANGEABLE HEADS, issued Oct. 3, 1989, to W. A. Buskirk et al. and 4,965,593, entitled PRINT QUALITY OF DOT PRINTERS, issued Oct. 23, 1990, to M. S. Hickman, the teachings of which are incorporated herein by reference.
Recently, full color inkjet printer/plotters have been developed which comprise a plurality of inkjet pens of diverse colors. A typical color inkjet printer/plotter has four inkjet pens, one that stores black ink, and three that store colored inks, e.g., magenta, cyan and yellow. The colors from the three color pens are mixed to obtain any particular color.
The pens are typically mounted in stalls within an assembly which is mounted on the carriage of the printer/plotter. The carriage assembly positions the inkjet pens and typically holds the circuitry required for interface to the heater circuits in the inkjet pens.
Full color printing and plotting requires that the colors from the individual pens be precisely applied to the media. This requires precise alignment of the carriage assembly. Unfortunately, mechanical misalignment of the pens in conventional inkjet printer/plotters results in offsets in the x direction (in the media or paper axis) and in the y direction (in the scan or carriage axis). This misalignment of the carriage assembly manifests as a misregistration of the print images applied by the individual pens. In addition, other misalignments may arise due to the speed of the carriage, the curvature of the platen and/or spray from the nozzles.
One conventional approach for aligning the pens involves the use of optical drop detectors. This technique is described and claimed in U.S. Pat. No. 4,922,270, issued May 1, 1990, to Cobbs et al. and entitled Inter Pen Offset Determination and Compensation in Multi-Pen Thermal Ink Jet Printing Systems, the teachings of which are incorporated herein by reference. The optical drop detectors detect the position of each ink drop as it leaves the pen. The system then calculates the point of impact of the drop on the print media. Unfortunately, the actual impact point often differs substantially from the calculated impact point due to angularity. Angularity results from the movement of the pen in the scan axis as ink is being ejected. That is, there is a delay between the time that the drop of ink is ejected and the time that the drop impacts the media. This flight time delay causes the drop to traverse an angular path toward the media. If not accurately calculated and corrected, this would cause a distortion in the print image. However, inasmuch as accurate calculation and correction has heretofore been difficult to achieve, this technique has been found to be inadequate for current product specifications for full color printing.
In another conventional approach, a test pattern is printed and the print image is sensed optically to determine the degree of image misregistration. This technique is disclosed and claimed in U.S. patent application Ser. No. 07/786,145, entitled Automatic Print Cartridge Alignment Sensor System, filed Oct. 31, 1991 by Robert D. Haselby (the teachings of which are incorporated herein by reference). However, this system is slow in that it required a self-calibration reference pattern for aligning the sensor.
Thus, there is a need in the art for systems and techniques for providing accurate image registration in multicolor, multi-pen inkjet printer/plotters.